In the field of motor vehicles, there are existing methods for detecting an identifier for starting a motor vehicle, which may be used to determine whether the identifier is inside the motor vehicle, with a margin not exceeding twenty centimeters around the motor vehicle, according to a Thatcham standard which is well known to those skilled in the art. Thus it is ensured that nobody can start the motor vehicle while the vehicle's user and his identifier are outside said vehicle. To provide this function, the motor vehicle comprises a first antenna which transmits a signal at low frequency (125 kHz), and the identifier comprises a second antenna which receives this low frequency signal. This low frequency signal can be used to delimit in a precise manner the areas inside and outside the motor vehicle, in order to determine whether the identifier is in fact located in the passenger compartment of the motor vehicle.
One drawback of this prior art is that, if it is desired to use a method compatible with identifiers such as cellphones using Bluetooth Low Energy™ technology, the motor vehicle must comprise a first antenna which transmits a Bluetooth Low Energy™ signal to communicate with said cellphones. With such technology, it is difficult to delimit different areas precisely, notably because of parasitic signals caused by reflections of the transmitted signals from the metal parts of the motor vehicle, these reflections being present for signals other than low frequency signals.